This invention relates to a method and a device for measuring light diffusely reflected from an optically nonuniform specimen, such as a reagent test strip used for qualitative and quantitative analysis of body fluids.
The spectral characteristics of light reflected by a specimen are related to the color of the specimen. With the advent of instrumental analysis, the color of a specimen under test has become one of the most widely used bases for biochemical assay procedures. For example, a reagent test strip used for qualitative and quantitative analysis can be contacted for a prescribed period of time with a body fluid, such as blood or urine. The reflectance spectrum of the colored test strip will vary depending on the concentration of the target substance in the body fluid being examined. Thus, by photoelectrically measuring light reflected from the test strip and computing the resulting reflectance, the desired analysis can be made by correlation of reflectance to concentration.
A reagent test strip is typically a test paper treated with various chemical reagents. Since the surface of the test strip does not have a uniform topography, it is difficult to make accurate and reproducible measurements because the result may depend on where the measurement is made on the strip or on measurement geometry. One solution to this problem has been to provide substantially uniform illumination of the test strip using an integrating sphere or hemisphere interposed between the light source and the strip. While this solution has met with some success, the integrating sphere or hemisphere adds to the cost of the device and may increase its size.
In addition, some illumination from the light source is dissipated or lost when an integrating sphere or hemisphere is utilized. Thus, in order to obtain a desired magnitude of reflected energy, the magnitude of the illumination must be correspondingly increased to compensate for the light dissipated in the device. This can require a larger light source and can lead to increased power consumption.
It has also been found with reagent test strips that the reflectance measurement is sensitive to the orientation of the strip in the test strip holder. For example, slight twisting or inclination of the specimen affects the magnitude of the reflectance measurement. In addition, it has been found that the reflectance measurement can be affected by the distance between the test strip and the light source.
Thus, there exists a need in the art for a method and a device for making rapid, accurate and reproducible light reflectance measurements from an optically nonuniform specimen under test. The device should employ direct illumination of the specimen to eliminate the need for an integrating sphere or hemisphere. The device should minimize the amount of illumination dissipated or lost between the light source and the specimen in order to minimize power consumption. The device should provide reflectance measurements that are less sensitive to subtle changes in the orientation of the specimen in the specimen holder and less susceptible to variations in the distance between the light source and the specimen. In addition, the device should be capable of compensating for changes in performance of components in the device, especially changes in illumination of the specimen.